What Are the Definitions of True Airspeed & Ground Speed?
True airspeed (TAS) is the actual speed of an aircraft as it travels through the air. It is corrected for temperature and pressure altitude. Installation of a true speed indicator is not typical but may be found on higher-performance aircraft.
Ground speed (GS) is the speed of an aircraft relative to the ground. It is corrected for wind. It decreases with a headwind and increases with a tailwind. There can be a significant change of ground speed in a crosswind scenario with a relatively small heading change.
Note that ground speed is measured horizontally, so if an aircraft climbs completely vertically, it would have a ground speed of zero.
How Can True Airspeed & Ground Speed Be Calculated?
You must know what your calibrated airspeed is to be able to calculate true airspeed.
This can be done by taking your indicated airspeed and referring to Section 5 of your Pilot Operating Handbook to calculate the air density against calibrated airspeed. You can also use an E6B flight computer. In both instances, accuracy is determined by accounting for pressure and temperature variation.
In the past, ground speed was calculated by taking note of checkpoints along your route and then dividing the distance by the time spent travelling between them. Today, ground speed can be calculated through the use of an inertial navigation system, GPS, or an E6B flight computer.
What Are True Airspeed & Ground Speed Used For?
True airspeed plays a couple of important roles in flight. It is vital for accurate navigation of an aircraft, and for flight planning purposes. True airspeed must be known to ensure accurate fuel burn and time estimates.
Ground speed is primarily used for performance during cross-country planning.